Hitherto, in an image forming apparatus for forming an image according to an electrophotographic method, a cartridge for supporting an image carrier such as a photosensitive drum on the surface of which an electrostatic latent image can be formed, and a cartridge for supporting a developing unit such as a developing roller or the like for developing the electrostatic latent image using a developer, or a cartridge composed of an integrated combination of the two cartridges (which are hereinafter generally referred to as an image forming cartridge) are structured such that they can be mounted onto and removed from the main body of the image forming apparatus. Since these image forming cartridges are structured so as to be mountable onto and removable from the apparatus main body, the image forming cartridge can be replaced according to the life spans of the photosensitive drum, developing roller and the like and thus the image forming apparatus can be used continuously.
Also, in this type of image forming apparatus, there is proposed a technology in which power is supplied through a slip ring to a cleaning electro-conductive brush for removing toner left on the surface of the photosensitive drum, and a shunt resistor for monitoring the power supply state is connected in series to an electric circuit reaching the slip ring (for example, see JP-A-9-28060). In such an apparatus, a meter relay, a sequencer and an alarm device in this particular order are connected in series to the shunt resistor and, when the output voltage of the shunt resistor falls below a normal level, a signal is sent to the alarm device.
Further, in this type of image forming apparatus, there is proposed a technology in which a voltage dividing resistor is connected to the power receiving terminal of the image forming cartridge. The divided voltage thereof is converted to digital data and is read into a CPU, where it is checked whether a voltage is applied to the power receiving terminal or not, that is, whether the power receiving terminal is connected to a high voltage supply terminal or not (for example, see JP-A-2001-83778).